Pitch axis control systems including both a pitch control system and a trim control system are adaptable to provide a plurality of control functions. One such function is to maintain level flight at a given attitude thereby relieving the pilot of the need to constantly adjust the pitch axis control elements to compensate for turbulence. In addition, a pitch axis control system enables the pilot to command an aircraft attitude other than horizontal. Pitch axis control systems are also made responsive to a glide slope coupler to automatically control aircraft approach for landing.
In most aircraft, the primary pitch control is subject to the greatest movement in order to obtain all the necessary flight attitudes. Nearly all aircraft are provided with a pilot actuated pitch control system which is manually adjusted during take-off, climb, approach and landing, and from time to time during normal flight as a result of changes in power settings, changes in the weight of the aircraft due to the use of fuel and shift of passengers, or when changing altitude. With an aircraft operating in a selected attitude the primary pitch control system is automatically adjusted from an autopilot system. Virtually all general aviation aircraft have a trim system which is a subsystem of the primary pitch control and is used to establish a desired pitch attitude without a force being applied to the primary pitch control surface by the pilot, or by the autopilot. The trim system usually accommodates all control changes necessary during normal flight. Proper adjustment of the pitch trim system materially reduces pilot fatigue when the aircraft is operated in the manual mode and improves the handling capabilities of the aircraft.
If an aircraft is not properly trimmed while being controlled by an automatic pilot, the autopilot will be required to continuously exert a corrective force on the controls. This, together with changes in the trim requirement during the course of autopilot flight, frequently results in relatively high and occasionally excessive loads on the autopilot. On occasion this results in an abrupt change in the attitude of the aircraft which can be quite disturbing to occupants of the aircraft and at best, is undesirable.
Additional convenience and safety have heretofore been provided by electrically operated pitch trim systems such as, for example, the one described in U.S. Pat. No. 3,936,715 issued Feb. 3, 1976 to John M. Nixon, et al. The system disclosed in this patent automatically trims the aircraft in response to a voltage applied to an integrated type servo motor coupled to the primary control surface.
Another automatic trim system which is available for aircraft trim control in flight is described in U.S. Pat. No. 4,281,811, issued Aug. 4, 1981 to John M. Nixon. This patent describes an automatic trim system that includes a control surface and a trim surface associated with the control surface for changing the effect of the control surface to maintain a desired flight attitude. The system includes a limiter responsive to an input signal that generates a limited trim control signal having a preselected maximum amplitude. The signal of limited authority varies up to plus or minus 20% of a normalized out-of-trim signal. The system includes a malfunction detector that identifies a trim system malfunction to generate a signal that disconnects the trim system from automatic operation.